This invention relates to methods and apparatus for combusting carbonaceous fuels, and more particularly to methods and apparatus for combusting carbonaceous fuels to produce a hot gaseous effluent for use a source of heat (e.g., in a furnace) or power (e.g., as a motive fluid in a turbine system).
In U.S. patent application Ser. No. 358,411, now U.S. Pat. No. 3,928,961, filed May 8, 1973, in the name of William C. Pfefferle and assigned to the same assignee as that of the present invention, and incorporated herein by reference, there is disclosed a process designated catalytically-supported, thermal combustion. According to this method, carbonaceous fuels can be combusted very efficiently and at thermal reaction rates in the presence of a solid oxidation catalyst at temperatures below nitrogen-oxide-forming temperatures. As described in application Ser. No. 348,411, this combustion method involves essentially adiabatic combustion of a mixture of fuel and air, or of fuel, air, and inert gases, in the presence of a catalyst operating at a temperature substantially above the instantaneous auto-ignition temperature of the mixture, but below a temperature that would result in any substantial formation of oxides of nitrogen. Essentially adiabatic combustion means that the operating temperature of the catalyst does not differ by more than about 300.degree. F., more typically no more than about 150.degree. F., from the adiabatic flame temperature of the mixture due to the heat losses from the catalyst. The instantaneous auto-ignition temperature of the mixture is defined herein and in application Ser. No. 358,411 to mean the temperature at which the ignition lag of the mixture entering the catalyst is negligible relative to the residence time in the combustion zone of the mixture undergoing combustion. Typically, the operating temperature of the catalyst is in the range from about 1,700.degree. to about 3,200.degree. F., preferably from about 2,000.degree. to about 3,000.degree. F. As pointed out in application Ser. No. 358,411, the combustion occurs under these conditions at a rate substantially higher than the conventional catalytic combustion rate. Combustion of the gases exiting from the catalyst zone may be substantially complete, or combustion may continue downstream of the zone containing the catalyst.
Various detailed systems have been devised which involve or utilize the process of catalytically-supported, thermal combustion to which application Ser. No. 358,411 is directed. Among these is the method of U.S. application Ser. No. 463,436, now U.S. Pat. No. 3,940,923, filed Apr. 24, 1974, in the name of William C. Pfefferle and assigned to the same assignee as that of the present invention. Application Ser. No. 463,436 discloses, among other systems, a system in which fuel from one source is mixed with compressed air to give a first mixture, which may have an adiabatic flame temperature of about 2,600.degree. F., and this fuel is burned in a catalyst bed giving an effluent having a temperature indicated to rise to 2,500.degree. F. downstream of the catalyst. A portion of the same fuel-air mixture is added to the catalyst effluent and burns homogeneously as it passes through a first turbine wheel shown to have a temperature of 1,900.degree.-2,000.degree. F. In one arrangement fuel from a second source is mixed with the compressed air to provide another fuel-air mixture (preferably in proportions providing a mixture similar to the first mixture), which is introduced downstream of the first turbine wheel and burns in a second turbine wheel, where temperatures of 1,800.degree.-1,700.degree. F. are shown. Whether or not additional fuel-air mixtures are fed downstream of the catalyst in these various systems, burning of uncombusted fuel values occurs in the expansion zone or zones of the turbine to counteract the cooling effect of expansion, and, in the examples given, the various mixtures appear to have combustion temperatures (before such cooling) which are about the same throughout the system.
Also, in systems not utilizing a catalyst and involving elevated combustion temperatures, additional fuel conventionally may be introduced downstream of the main burner, as in jet engine reheat or afterburner systems. Illustrating a related system, the British Pat. No. 941,830 of the General Electric Company shows a conventional combustor for supplying a gas turbine engine. Primary fuel in the form of a kerosene-gasoline mixture is introduced through a nozzle to the dome at the upstream end of the combustor, and compressed air passes through openings in the combustor shell to swirl back into the dome, where combustion occurs at high temperatures, probably in the neighborhood of 4,000.degree. F., as the air meets fuel vapors from the droplets leaving the nozzle. To permit burning some higher-energy fuel to increase turbine or jet engine performance, a borohydride fuel (which may contain hydrocarbons) is introduced just downstream of the point where the air circulates upstream. This permits burning of the borohydride in the downstream portions of the combustor without fouling the main nozzle and the relatively quiescent dome area with solid oxide deposits from the borohydride, and also without exposing the combustion liner to the extreme combustion temperature of high energy fuel since more cooling air is admitted to the downstream portions of the combustor.